1. Field of the Invention
The present invention relates generally to electronic circuits, and, more particularly, to a method and system to drive large off-chip loads, such as, for example, laser diodes.
2. Art Background
Nowadays, the design of modern very large scale integration (“VLSI”) systems is performed with a view towards lowering manufacturing costs. Complementary metal oxide semiconductor (“CMOS”) device technology is particularly attractive, since it is relatively low-cost, has high-performance, and enables the integration of greater device functionality on a single integrated circuit chip. Although CMOS technology scales well, some of the system blocks that interface the integrated circuit (“IC”) with external devices are difficult to integrate because the necessary output signals levels may not scale as well.
Such high integration of functionality on a single integrated circuit chip is exemplified in a typical 10-gigabit Ethernet chip, which includes an extension attachment unit interface (XAUI), a packet coding sublayer (PCS), a limiting amplifier, a clock and data recovery and demultiplexer module, as well as a 10 Gigabit/second (Gbps) multiplexer module. The 10 Gbps multiplexer module further interfaces with an external buffer that drives the inputs of an external load, such as, for example, a laser diode. The photonic emission of the laser diode is modulated, sending signals down an optical fiber. The voltage swing requirement to modulate these laser diodes may be stringent, requiring the use of a fairly large driver device.
For most applications today, a buffer device, integrated with the IC, interfaces with an off-chip laser driver. The single-ended input impedance is typically a standard value, such as 50 Ohms (50Ω). The output of the off-chip laser driver is designed appropriately for any given application in order to drive a certain diode load. For some applications, the laser diodes may be driven single-ended, with the diode biased appropriately to provide a 50Ω termination. Alternatively, the diodes may be driven differentially. For a differential configuration, the diode may be again biased to provide a 50Ω termination, which is equivalent to a single-ended termination of 25Ω.
Since the prior applications involve a high overall system cost, in part due to the cost of the off-chip driver device, what is needed is an integrated circuit that would eliminate the cost of the off-chip laser driver device by integrating an on-chip driver device capable of directly driving an external diode load. In addition, what is needed is an integrated circuit that would be capable of driving both a single-ended 50Ω load, as well as a differential 50Ω load, equivalent to 25Ω single-ended, thus allowing for greater flexibility, depending on the current application.